1. Field of the Invention
This invention relates, in general, to pedestals for tracking antenna and more particularly to satellite tracking antenna pedestals used on ships and other mobile applications and methods for their use.
2. Description of Related Art
The invention is especially suitable for use aboard ship wherein an antenna is operated to track a. transmitting station, such as a communications satellite, notwithstanding roll, pitch, yaw, and turn motions of a ship at sea.
Antennas used in shipboard satellite communication terminals typically are highly directive. For such antennas to operate effectively they must be pointed continuously and accurately in the direction toward the satellite.
When a ship changes its geographical position, or when the satellite changes its position in orbit, and when the ship rolls, pitches, yaws and turns, an antenna mounted on the ship will tend to become misdirected. In addition to these disturbances the antenna will be subjected to other environmental stresses such as vibrations caused by shipboard machinery and shocks caused by wave pounding. All of these effects must be compensated for so that the antenna pointing can be accurately directed and maintained in such direction.
For nearly two decades, Sea Tel, Inc. has manufactured antenna systems of the type described in U.S. Pat. No. 5,419,521 to Matthews. Such antenna systems have a three-axis pedestal and employ a fluidic tilt or fluidic level sensor mounted in a structure referred to as a “Level Platform” or “Level Cage” in order to provide an accurate and stable Horizontal reference for directing servo stabilized antenna products. For example, the '521 patent shows a level platform (45) and a fluidic tilt sensor (54) which are illustrated in FIGS. 3 and 7A, respectively.
The fluidic tilt sensor produces very stable tilt angle measurements with respect to earth's gravity vector, but only over a limited angular range of +/−30° to +/−40°. As an antenna system's pointing angle can change from 0° to 90°, however, such fluidic tilt sensors can not be mounted directly to the antenna, Instead, the fluidic tilt sensor must be mounted in a structure that is rotated opposite the antenna pointing angle so that the structure always remains in an attitude that is substantially level with respect to the local horizon and perpendicular to earth's gravity vector. For example, an as shown in FIG. 1, a fluidic tilt sensor may be mounted within level platform structure 20 that is rotated opposite the antenna pointing angle by a level platform drive motor 22 via a drive belt 23 or other suitable means.
In addition to the fluidic tilt sensor for the elevation axis, the level platform structure normally incorporates a second fluidic tilt sensor for the cross-level axis and three inertial-rotational rate sensors. While the level platform design works very well, the configuration of the level platform structure adds to the complexity and cost of the antenna system. Namely, as shown in FIG. 1, the level platform structure 20 itself, the bearings which rotatably support hold the structure, the drive motor 22, the drive belt 23 and associated pulleys and hardware to rotationally drive and support the structure adds significant complexity and costs to the overall antenna system. In addition, electrical harnesses 25 connecting the drive motor to the level platform structure essentially sits in an outdoor environment near radar equipment, and the harnesses must be braided with shielded cable further adding significant costs.
A low cost and stable gravity reference sensor having a minimum range of 0 to 90°, plus the expected Tangential Acceleration range of +/−30 to +/−45 degrees is desired.
It would therefore be useful to provide an improved pedestal and control assembly for a tracking antenna having improved means to provide a simplified level reference assembly to overcome the above and other disadvantages of known pedestals.